The developing fetus normally receives the majority of its nutrients through the fetal placental circulation. Nutrients are also absorbed by the fetal gastrointestinal tract from swallowed amniotic fluid. When placental function is inadequate to meet the needs of the growing fetus, nutritional supplementation through the digestive tract may be possible. Using a chornic sheep preparation, with indwelling fetal and maternal catheters, we are evaluating the quantitative contribution of gastrointestinal absorption to fetal nutrition and to what extent this can safely be increased in the normal and compromised fetus. Gastrointestinal and umbilical uptake of glucose, fructose, amino acids, lactate, acetate and oxygen are being determined from measurements of the concentration of these substances in fetal blood entering and leaving the gastrointestinal tract and placenta and from measurements of blood flow to these organs. The relative importance of these two routes in supplying the fetus with nutrients is being compared. The extent to which gastrointestinal uptake can be increased is being evaluated by infusing nutrients into the fetal intestine and rumen. Fetal oxygen consumption, acid-base balance, blood urea nitrogen and ammonia concentrations are being measured when gastrointestinal absorption is increased. Once determined, the response of the normal fetus will be compared to that of fetuses made hypoxemic by having their mothers breathe 10% oxygen and to that of fetuses experimentally growth retarded.